In recent years, techniques for enhancement of function and miniaturization in mobile terminal units have made startling progress. However, to enhance the performance of an antenna unit, which is a major element of a mobile terminal unit, and to miniaturize it, are difficult, compared with other circuit techniques or device techniques. This is an obstacle in proceeding to miniaturize the entire mobile terminal unit. A pager, which is now the smallest one of mobile terminal units, is used mostly as it is attached to a human body. Therefore, the antenna is desired to have a high gain even when being attached to the human body. Because of this, many pagers use a loop antenna that is one of magnetic-field type antennas whose antenna gain is not deteriorated when even when being attached to the human body.
However, by the reason that the gain of loop antenna varies in proportion to the opening area of loop, it is difficult to obtain a loop opening area needed to have a gain equal to that of a larger pager when the miniaturization of pager is required. Also, with the miniaturization of pager, the distance between the loop antenna and other parts becomes short. Therefore, there is another problem that the antenna gain of loop antenna is often deteriorated because it is likely to be affected by nearby metal parts.
Japanese patent application laid-open Nos.7-3211688(1995) titled "Built-in Antenna for Radio Unit" and 61-123303(1996) titled "Antenna for Miniature Radio Unit" disclose a technique that a cell-encasing part or cell itself is used to minimize the antenna unit of a portable radio unit as a mobile terminal unit. The former is the invention that part of a cell attachment part in the cell-encasing part is used as a loop antenna. On the other hand, the latter is the invention that a shielding case in the radio section of portable radio unit is used as an earth plate and a cell for feeding power source in the cell-encasing part shielded from the radio section is an antenna element.
The antennas disclosed above aim to minimize the size by using the space for the cell-encasing part. However, such a technique cannot meet further requirements as to the miniaturization and the enhancement of performance any more because the requirements to mobile terminal unit or portable radio unit are rapidly increased in recent years.